1. Field of the Invention
The present invention relates generally to fluid delivery systems, and more particularly, to improved fluid delivery systems and methods that are suitable for the direct application of ultra-pure fluids to a wafer surface in processes such as CMP, photolithography and dielectric processes.
2. Description of Related Art
Many industries such as semiconductor, pharmaceutical, and bio-technology experience fluid delivery problems due to the typically low flow rates, the use of abrasive and aggressive chemical fluids, and the need for contaminant free, accurate, compact, and real-time fluid delivery and/or blending systems.
Generally the semiconductor industry uses the word “tool” to refer to a system comprised of pieces of hardware that perform an operation on a wafer surface. These operations include the creation of thin films with some form of chemical deposition or fluid coating process, the etching of films in precise patterns, the polishing of films to reduce deposited films to a planar surface and the cleaning of surfaces to remove unneeded films, particles and chemical contaminants. The tool can either be stand alone or incorporated with other tools in a cluster structure where multiple sequential operations on the wafer are done in one location.
For example, Chemical-Mechanical Planarization (CMP) is a critical process in the semiconductor industry that involves a process to flatten the wafer surface of a semiconductor by applying an ultra-pure fluid containing small abrasive particles and a reactive agent between the wafer surface and a polishing pad. In most applications, the polishing pad rotates at a controlled speed against the wafer to flatten the surface. Over-polishing the wafer can result in altering or removing critical wafer structures. Conversely, under-polishing of the wafer can result in an unacceptable wafer surface. The polishing rate of the wafer is highly dependent upon the delivery rate of the fluid and the total amount of fluid delivered during a polishing operation.
In addition to fluid flow rate, prevention of contaminants in the fluid being applied to the polishing pad is critical. It is a problem in some applications that the instruments used to control the process are a source of contaminants to the process material. This is undesirable for use in systems where material an ultra high of purity must be delivered to a user's application. For example, the metal flow tube of a typical Coriolis flowmeter can be a source of contaminates. This is the case in the fabrication of semi-conductor wafers, which requires the use of material that is free of contaminants including ions migrating from the flow tube wall. This released material can cause the chips on a semi-conductor wafer to be defective. The same is true for a glass flow tube, which can release the leaded ions from the glass into the material flow. The same is also true for the flow tubes formed of conventional plastics.
Because prevention of ionic contaminants is critical, the majority of CMP processes utilize peristaltic pumps along with a suitable high purity tubing material to supply fluid to the polishing pad. While the clean flow path non-intrusive characteristic of peristaltic pumps is acceptable for CMP applications, the use of these pumps to attempt to control fluid flow rate is very inaccurate since they are operated open-loop with no flow measurement feedback
Another process used in the semiconductor industry requiring accurate control of fluid flows and a contaminant free environment is the photolithography process. As is known in the art, photolithography is a process that applies a light sensitive polymer, known as resist, to the wafer surface. A photomask containing a pattern of the structures to be fabricated on the wafer surface is placed between the resist covered wafer and a light source. The light reacts with the resist by either weakening or strengthening the resist polymer. After the resist is exposed to light, the wafer is developed with the application of fluid chemicals that remove the weakened resist.
A modification of this process applies a host of new liquids to the wafer surface to create films that will become an integral part of the final semiconductor. The primary function of these films is to act as an insulator between electrical conducting wires. A variety of “spin-on” materials are being evaluated with a wide variety of chemical compositions and physical properties. The key difference between the lithography process and the spin-on deposition is that any defect in the film (such as a void, bubble or particle) is now permanently embedded in the structure of the semiconductor and could result in non-functioning devices and a financial loss for the semiconductor producer.
Both of these processes take place in a tool called a “track.” The purpose of the track is to apply a precise volume of fluid to the surface of a stationary or slowly spinning wafer. After the liquid application, the wafer rotation speed is rapidly increased and the liquid on the wafer surface is spun off the edge. A very thin, consistent thickness of liquid remains from the center of the wafer to the edge. Some of the variables that affect liquid thickness include the resist or dielectric viscosity, solvent concentration in the resist or dielectric, the amount of resist/dielectric dispensed, speed of dispense, etc.
The track will also provide additional processing steps after liquid application that changes the liquid to a polymer using a bake process that also removes any solvent in the film. Dielectric films can also be exposed to other chemical treatments to convert the liquid film to the proper solid structure. The track also controls the environment around the wafer to prevent changes in humidity or temperature and chemical contaminants from affecting the performance of the film. Track system performance is determined by the accuracy and repeatability of liquid delivered to the wafer surface in addition to minimizing defects in the film caused by voids, bubbles and particles.
Problems associated with currently available fluid delivery systems used in processes such as CMP and spin-on applications include the inability to provide closed loop flow measurement, the inability to provide an accurate fluid delivery rate based on changing head pressure of the pump system, variances in the volume of tubing used in the pump, and pulsations from the pump. Further, periodic weekly or even daily calibration of the pump may be required. Other problems associated with current fluid delivery systems involve contamination of the fluid from shedding tubing particles.
Additional factors typically important in these industries include the need for real-time fluid property data such as flow rate, fluid temperature, viscosity, density and pressure. While all of the foregoing fluid properties can be measured using a combination of various instruments such as differential pressure transmitters, viscometers, densitometers, pressure transmitters, temperature elements or a combination of the instruments and a control system to calculate the fluid property values, the use these instruments can be expensive, have significant space requirements, require increased maintenance, and provide a greater potential for fluid leakage and process contamination. Therefore, there is a need for an efficient, compact and contaminant free solution to fluid delivery systems in the foregoing industries.
In other processes there is an increased need for a real-time blending system of multiple fluids requiring a high-purity flow path. In addition, blending based on a volumetric basis is generally unacceptable since typical blending formulas are based on molar ratios. Current blending methods include adding multiple fluids to a container on a weigh scale in an off-line manner, as shown in FIG. 1. Multiple fluids, A through N, flow into a container 11 placed on a scale 12. One fluid is allowed to run through a flow valve 13 at a time. The scale total is examined and when the desired amount of Fluid A has been added, the valve 13 is closed. The same process is repeated with the remaining fluids. Eventually, a total mixture is obtained. If too much or too little of any fluid has been added the process must continue until the proper mass of each fluid, within some acceptable error band, has been added.
Another known approach uses a level sensor to measure the volume of each fluid of the blend as it is being added to the vessel. This requires a very precise knowledge of the volume of the vessel with small increments of vessel height.
Unfortunately, the current batch production method can result in too much or too little of the final product being available when needed. Since having too little product available would shut a process down, extra product is always produced, meaning some product will be left over and not used. Since these products often have a limited shelf life (e.g. several hours) this excess product must be disposed of. This disposal is costly for several reasons. The product is typically uses very expensive chemicals, and the fluid mixture can often be very hazardous meaning it must be disposed of in a controlled and costly manner.
As technology develops, the need for manipulation of the blending formulas based on the differences in the product requirements, and additional new material components continues to increase, thus requiring a greater need for flexible, accurate and contaminant free real-time continuous blending systems. Another important factor includes the need for accurate pressure control to ensure proper blending and accurate fluid flow rates to the processing tool.
Thus, there is a need for fluid delivery systems that address shortcomings associated with the prior art.